The metallurgy of lead has been very well researched and developed from Roman times forward. Pyrometallurgical techniques were used in ancient England to refine lead and desired alloys, as disclosed in A Study of Lead Softening, Vineburg, Daryl Geoffrey, Master's Thesis. McGill University, Montreal, Canada (2003). The known art of refining advanced with the advent of the process patented by Henry Harris, et al, in London, England ISD 1922, as disclosed in Harris Process, Jones, T. D., ASARCO, The Wisconsin Engineer Volume 33 Number VII (1929). The “Harris process” used slag composition manipulation during pyrometallurgical processing to selectively remove impure compounds and elements found in lead bullion. The Harris process provides an environment where lead impurities including antimony, arsenic, tin, tellurium, and selenium are oxidized out of the lead by mixing or otherwise contacting the molten lead bullion with mildly oxidizing slags consisting of alkali metal hydroxides and other salts. The oxidizing power of the slag is then enhanced by use of air, or other oxidizing agents, such as alkali hydroxides mixed with alkali nitrates. After the alkali slag is sufficiently laden with impure metal hydroxides and compounds, the slag is decanted or otherwise removed from the lead.
By 1922, The American Smelting and Refining Company (“ASARCO”) had adapted the Harris process to use slag as a vehicle for initiating the refining operation at the ASARCO refinery at Perth Amboy, N.J. U.S. Pat. No. 2,113,643 (Betterton, et. al.) details the use of chloride slag mixtures to recover impurities from the refining of lead bullion. Betterton added gaseous chlorine to the molten slag to provide the oxidizing power to drive the impurity level in the slag to optimum levels. While not stated in the Betterton patent, the volatility of the various chlorides, particularly arsenic trichloride requires a gas handling system. Betterton adjusted the composition of the molten slag, consisting of sodium chloride, calcium chloride, magnesium chloride, and potassium chloride to produce a very low meting point. As the slag becomes loaded with impure metal chlorides, the viscosity and melting point change, thus providing the operator with convenient control parameters.
More modern processes include the KIVCET process where slag oxidation/reduction control is accomplished in the furnace rather than in the refining kettles. As the twentieth century drew to a close, costs and environmental considerations changed the complexion of lead refining and alloy production.
Practically speaking, the only waste products that can be economically disposed to the environment are: very low lead content iron/lime/silica blast furnace slags that must pass the EPA TCLP test; very clean alkali salts such as chlorides, sulfates, or carbonates; and very limited amounts of sulfur dioxide (released to the atmosphere). In many cases, the air discharge limits on sulfur dioxide are so low that conversion to marketable commercial sulfate solution or salt is necessary.
The United States secondary lead smelting industry is subject to environmental restrictions regarding discharge levels of lead and other toxic metals. Consequently, the industry uses reagents that have minimal impact on the discharge levels of toxins into the environment. Such reagents include air and oxygen. The use of air or oxygen for lead bullion refining has a very low initial cost. However, the process requires a very hot kettle at 575° C. to 650° C. (1000° F. to 1200° F.), which consumes more fuel and shortens kettle life. The process is slow and the by-product lead oxide containing the antimony, tin, arsenic and other elements consumes eight to twelve percent of the lead in the kettle. Lead loss, added energy costs, and shortened kettle life make the process expensive. In addition, tons of fluffy lead oxide powder forming on top of the refining kettle must be removed manually. This lead oxide by-product is an environmental hazard and as such, is strictly regulated by the EPA and OSHA.
Due largely to the high temperatures involved during conventional refining, a significant amount of antimony is removed from the lead along with other impurities. This can be undesirable in that antimony contributes to the structural strength of the lead alloys, and enhances casting with lead by improving the capacity of the molten lead to fill voids in the molds. Current production processes and practices using air for kettle refining exhibit high energy cost, lead-in-air regulatory compliance issues, long processing times (more than eight hours), and a nominal ten percent loss of product to the recycle loop for every kettle treated.
There is a need to improve the traditional lead refining process and rectify problems associated with impurity separation. There is also a need for more efficient removal of tin and improved retention of antimony when removing other impurities.